This invention relates to a circuit for generating signal voltages useful in the control of a discharge lamp, and more particularly useful for controlling the power to a high-pressure discharge lamp operating as a headlight or fog lamp in a motor vehicle or the like.
In U.S. Pat. No. 4,724,360, issued Feb. 9, 1988, and hereby incorporated by reference, there is disclosed a circuit for operating a high-pressure discharge lamp (e.g. a high-pressure metal halide lamp) from a source of DC voltage, such as an automobile battery, by means of a direct voltage to alternating voltage converter. The converter-ballast circuit described therein is especially useful for operating a headlight in an automobile or similar vehicle which uses a high-pressure discharge lamp as the headlight.
In a ballast system of the general type described in the aforesaid U.S. patent, it is necessary to control the open circuit voltage in order to insure that there is sufficient voltage available to produce reliable lamp ignition. It is also required to control the "run-up" current (i.e. the current delivered to the discharge lamp during the low arc voltage period that occurs prior to the steady state or normal lamp operating condition), and more importantly to control the steady state lamp power.